Mad cow disease facts

Mad cow disease is caused by an infectious transmissible agent termed a prion.

Mad cow disease was first noted in the 1970s and is thought to be related to another prion-caused disease termed scrapie that occurs in sheep; the first major outbreak of mad cow disease was in the United Kingdom, where more than 184,500 cases have been noted in cattle (2010 U.K. data found only 11 infected cattle).

In the United States, four cattle have been identified as having been infected with mad cow disease prions, while in Canada, 20 cases have been identified.

Two strains of the mad cow disease prion have been identified, typical BSE strain and atypical BSE strain; the atypical strain may only occur spontaneously.

Cattle-feed bans that prohibit feeding cattle any foods that contain cattle-derived or human-derived compounds may help reduce or eliminate transfer of typical BSE strain prions to other cattle.

The prevalence of mad cow disease in the U.S. and Canada is considered to be very low (about three to eight cases per million cattle in Canada and 0.167 cases per million in the United States); however, some researchers suggest these numbers are actually 18- to 48-fold higher.

About BSE

BSE (bovine spongiform encephalopathy) is a progressive neurological disorder of cattle that results from infection by an unusual transmissible agent called a prion. The nature of the transmissible agent is not well understood. Currently, the most accepted theory is that the agent is a modified form of a normal protein known as prion protein. For reasons that are not yet understood, the normal prion protein changes into a pathogenic (harmful) form that then damages the central nervous system of cattle.

Research indicates that the first probable infections of BSE in cows occurred during the 1970's with two cases of BSE being identified in 1986. BSE possibly originated as a result of feeding cattle meat-and-bone meal that contained BSE-infected products from a spontaneously occurring case of BSE or scrapie-infected sheep products. Scrapie is a prion disease of sheep. There is strong evidence and general agreement that the outbreak was then amplified and spread throughout the United Kingdom cattle industry by feeding rendered, prion-infected, bovine meat-and-bone meal to young calves.

The BSE epizootic in the United Kingdom peaked in January 1993 at almost 1,000 new cases per week. Over the next 17 years, the annual numbers of BSE cases has dropped sharply; 14,562 cases in 1995, 1,443 in 2000, 225 in 2005 and 11 cases in 2010. Cumulatively, through the end of 2010, more than 184,500 cases of BSE had been confirmed in the United Kingdom alone in more than 35,000 herds.

There exists strong epidemiologic and laboratory evidence for a causal association between a new human prion disease called variant Creutzfeldt-Jakob disease (vCJD) that was first reported from the United Kingdom in 1996 and the BSE outbreak in cattle. The interval between the most likely period for the initial extended exposure of the population to potentially BSE-contaminated food (1984-1986) and the onset of initial variant CJD cases (1994-1996) is consistent with known incubation periods for the human forms of prion disease.

Overview of BSE in North America

Through February 2015, BSE surveillance has identified 24 cases in North America: 4 BSE cases in the United States and 20 in Canada. Of the 4 cases identified in the United States, one was born in Canada; of the 20 cases identified in Canada, one was imported from the United Kingdom (see figure above).